Catalytic dehydrogenation as a commercially viable route to the production of olefinic hydrocarbons continues to grow in importance. C.sub.3 and C.sub.4 olefins have traditionally been recovered as by-products from either the steam cracking of natural gas, petroleum naphtha and gas oils in petrochemical plants, or from fluid catalytic cracking of petroleum gas oils in petroleum refineries. The production of the C.sub.3 and C.sub.4 olefins by these traditional methods has been largely non-selective, although some shifting of by-product yield can be obtained by changing feedstock composition or the cracking severity. However, the yields of specific C.sub.3 or C.sub.4 olefins, particularly monoolefins, are low and these changes may affect the yields of the primary products which may not be desirable.
When the desired production of a particular C.sub.3 or C.sub.4 olefin is required, a highly selective process is preferred. Catalytic dehydrogenation technology represents an economical method to produce propylene and butenes as primary products or in petrochemical complexes to alter the product distribution toward more profitable products. Products such as polymer grade propylene from propane, or normal and/or isobutenes from butane may be produced as commodity chemicals or converted into high value chemical products. For example, butanes can be converted into ethers such as methyl tertiary butyl ether (MTBE) in a complex comprising catalytic dehydrogenation, etherification and butane isomerization processes.
One example of a catalytic dehydrogenation process is taught in U.S. Pat. No. 4,381,417 to Vora et al. and is herein incorporated by reference. Vora et al. teach a process for the catalytic dehydrogenation of low molecular weight paraffinic hydrocarbons wherein the hydrogen-rich vapor streams resulting from each of the two vapor/liquid separations steps are recombined with the reactor effluent prior to compression, cooling and drying of the combined reactor effluent and hydrogen-rich stream. Because the light paraffins are relatively volatile, a more complicated separation scheme and a bulk condensation is normally required to effect the separation of the product olefins from the light by-products and hydrogen which are simultaneously produced in the process. It is therefore believed that U.S. Pat. No. 4,381,418 (Gewartowski et al.) is pertinent for its teaching of a catalytic dehydrogenation process for C.sub.2 + normally gaseous paraffinic hydrocarbons and the recovery of the products of the reaction. U.S. Pat. Nos. 4,430,517 and 4,486,547 issued to Imai et al. and U.S. Pat. No. 4,469,811 issued to Lucien are believed pertinent for their teaching of catalysts and operating conditions which can be employed for the dehydrogenation of low molecular weight paraffins.
Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more-strongly adsorbable gas can be an impurity which is removed from the less-strongly adsorbable gas which is taken off as product; or, the more-strongly adsorbable gas can be the desired product, which is separated from the less-strongly adsorbable gas. For example, it may be desirable to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desirable to recover more-strongly adsorbable gases, such as ethylene, from a feed to produce an ethylene-rich product.
In pressure swing adsorption, a multi-component gas is typically fed to at least one of a plurality of adsorption beds at an elevated pressure effective to adsorb at least one component, while at least one other component substantially passes through. At a defined time, feed to the adsorber is terminated and the bed is depressurized by one or more co-current to the direction of feed depressurization steps wherein pressure is reduced to a level which permits the separated, less-strongly adsorbed component or components remaining in the bed to be drawn off without significant removal of the more strongly adsorbed components. Then, the bed is depressurized by a countercurrent depressurization step wherein the pressure on the bed is further reduced by withdrawing desorbed gas countercurrently to the direction of feed. Finally, the bed is purged and repressurized.
U.S. Pat. Nos. 5,227,567 and 5,177,293 to Mitariten et at. teach processes for the separation and recovery of a product stream from a dehydrogenation reaction wherein a chiller system is employed to concentrate the olefin products and heavy hydrocarbons and a pressure swing adsorption unit is employed to separate a pure hydrogen stream from the heavy hydrocarbons. In U.S. Pat. No. 5,177,293, the desorbed hydrocarbons from the PSA unit are returned to the chiller and a portion of the pure hydrogen is returned to the dehydrogenation reactor. In U.S. Pat. No. 5,227,567, the desorbed hydrocarbons from the PSA unit are admixed with the reactor effluent from the dehydrogenation reactor, compressed, and charged to the chiller, while a potion of the hydrogen-rich gas from the PSA zone is returned to the dehydrogenation reactor. In both of these schemes, the capacity of the overall dehydrogenation system is limited by maximum vapor phase velocities within the dehydrogenation reactors. During the dehydrogenation reaction, by-products of the reaction tend to build up in the reactor/chiller system, thereby reducing the capacity of the overall dehydrogenation system and the efficiency of the separation of the dehydrogenation products.
Processes are sought which remove by-products of the reaction from the products of the reaction without allowing these by-products to build up in the recovery sections of the plant.